A very common application field for position measuring devices is numerically controlled machine tools. In that case, they are used for determining actual positional values that are required by sequential electronics, e.g., a numerical control, for calculating setpoint values for control loops, by which the feed of a tool or a work piece is controlled. Rotary transducers or angle-measuring devices, for example, are directly or indirectly coupled to the shaft of a motor for this purpose, and linear measuring devices, for example, to a tool carriage that can be moved.
Modern position measuring devices generate digital absolute measuring values. These may be both positional values and measured values which are derived from temporal characteristics of positional values, such as speed values or acceleration values. Moreover, position measuring devices which additionally ascertain measured values from sensors that are situated inside or outside of the position measuring device are conventional, e.g., temperature or vibration sensors. The transmission of the measured values from the position measuring device to the sequential electronics takes place via digital data transmission interfaces. European Published Patent Application No. 0 660 209 describes a position measuring device having a digital data transmission interface. Complex analog and digital circuit blocks are required to provide the necessary functionalities, e.g., the acquisition of positional signals, processing of the positional signals into digital positional values or measured values derived therefrom, and the communication with the sequential electronics.
A central unit in position measuring devices is a clock generator, which generates a clock signal that is used as time base for many functional blocks of the position measuring device, such as for A/D converters, finite state machines, the digital data transmission interface or possibly a microprocessor or microcontroller as part of a central processing unit. If the frequency of the clock signal changes because of a malfunction, it may happen that functional blocks are operated outside their specification, which in turn may lead to measuring mistakes, sporadic downtimes, etc.
Some of the errors caused in this manner, especially if they result in faulty measuring values, may not be detected in the sequential electronics. In unfavorable cases, for example, the currently processed work piece in a machine tool may become unusable or the machine tool may even suffer damage as a result. Even worse than the possible financial loss is the injury risk that may arise for the operators. It is therefore important to ensure the proper functioning of the clock generator.